In recent years, attention is paid to an adaptive modulation and communication system (sometimes referred to as AMCS communication system hereunder). The adaptive modulation and coding communication system features in varying the coding rate indicating the ratio of the actual data (user data) and error correction codes for the actual data and the multi-level modulation factor in response to the quality of the transmission line. In high quality transmission lines, high speed data transmission is possible in sacrifice of noise resistance characteristic. On the other hand, in lower quality transmission lines, noise resistance characteristic is improved in sacrifice of data rate.
The AMCS communication system is applied to, for example, an EGPRS (Enhanced General Packet Radio Service) used in a GSM (Global System for Mobile Communications) and a wireless communication system such as an HDR (High Data Rate) developed by Qualcomm Corporation. Furthermore, the AMCS communication system is planned to be utilized in the W-CDMA (Wide Band Code Division Multiple Access) system which is forecasted to gain popularity in future.
Incidentally, the AMCS communication system may be applied to communication between, e.g., mobile terminals as movable stations and a base station as a fixed station. In the AMCS communication system, mobile terminals send to the base station a receiving quality message indicating their receiving quality and the base station determines the modulation coding mode based on the receiving quality message transmitted from the mobile terminal 1.
However, there is considerable delay between a mobile terminal determining its receiving quality and the base station recognizing it because the base station must receive the receiving quality message and demodulates and decodes such message.
Accordingly, at the time when the base station recognized the receiving quality indicated by the receiving quality message, the current receiving quality of the mobile terminal might have changed. This means that the optimum modulation coding mode cannot be chosen, thereby degrading the transmission efficiency.
Such problem is more significant especially when the mobile terminal user is moving in a high speed train or the like, thereby encountering very rapid change in characteristics of the transmission path.
On the other hand, from the viewpoint of improvement of the transmission efficiency, it is preferable to increase the period of transmission of the receiving quality message of the mobile terminal. However, in this case, the current receiving quality of the mobile terminal may largely depart from the receiving quality indicated by the receiving quality message, thereby adversely affecting the improvement of the transmission efficiency by the adaptive modulation coding.